(ITS#4645) play cleanup handlers when abandoning

Full_Name: Pierangelo Masarati
Version: HEAD,all
OS: irrelevant
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (87.28.220.33)
Submitted by: ando
When an operation is abandoned, some backends (including the proxies) do not
call any of the send_ldap_*() functions, so callback cleanup handlers don't get
called. This may lead to resource leaks and other side effects. I've fixed
this in back-{ldap,meta}/search.c, but there might be more.
p.